1. Field of the Invention
The present invention relates generally to displays and their associated row driving circuits and in particular to LCD displays using redundant schemes to compensate a damaged row select driver subcircuit. In addition, the present invention relates to displays that compensate for damaged row select lines within a display area.
2. Description of Related Art
An active matrix liquid crystal display device commonly comprises a plurality of pixel elements arranged in a matrix of rows and columns and supported on one side with a thin layer of electro-optic material.
Each picture element consists of a switching transistor, a liquid crystal cell, that can be modeled electrically as a capacitor known as a pixel capacitor, and an optional storage capacitor. One of the electrodes of the pixel capacitor is on one side of the matrix display and a common electrode for all the pixel capacitors is formed on the opposite side of the matrix display. The pixel element, including a liquid crystal cell and an optional storage capacitor, is driven by a thin film transistor (TFT) that is used as a switching device. The pixel transistor is used for transferring an input data signal to the corresponding pixel element.
The resolution of a picture is defined by the number of pixels in the display area. In a conventional unscanned LCD display with X columns and Y rows, the display requires X plus Y column and row driver leads. Therefore, it is clear, in the case of an unscanned LCD display that the greater the number of columns and rows in a display area the more input leads are required to connect external signals to the LCD display. This problem of unusually large numbers of external display leads has been addressed in the past. One solution has been to integrate a driver circuit directly onto the display substrate. This reduces the number of external input leads and driving chips significantly.
An integrated row select driver circuit on an LCD display substrate is disclosed in pending U.S. patent application Ser. No. 07/996,979 filed Dec. 24, 1992, entitled "A Select Driver Circuit for an LCD Display", now U.S. Pat. No. 5,313,222, and naming Sywe N. Lee as inventor and is hereby incorporated by reference. The select driver circuit disclosed in the above referenced patent application includes a plurality of row select driver subcircuits corresponding to the number of pixel row lines. An output of each of the row select driver subcircuits is connected to a corresponding pixel row line and to a successive row select driver subcircuit as an activating input. The basic block diagram of the display system that includes a display device having a TFT array in an integrated driver circuit formed on the display along with a peripheral control/drive circuit is shown in FIG. 1. FIG. 2 shows a more detailed circuit diagram for the row select driver circuit and its corresponding subcircuits with the external control and driver signals. FIGS. 2 discloses a circuit diagram wherein all even numbered stages include VSS.sub.y and all odd numbered stages or rows include VSS.sub.x. The above referenced pending patent application also describes a circuit diagram wherein all stages are supplied with VSS.sub.x and would then require one less external lead to be connected to the row select driver circuit. In a case wherein a display has 384 columns and 240 rows there would be 92,106 pixels formed.
Transistors forming the row select driver circuits are usually thin film transistors deposited on the same substrate as the display pixel array. As shown in FIG. 2, only eleven external leads are required to connect the row select driver circuit in order to control 240 leads from each of the rows of the LCD display. This is in stark contrast to the 240 leads that would be required on a conventional unscanned LCD display. This reduction in the number of lead connections significantly simplifies the LCD assembly and packaging. In manufacturing an integrated row select driver circuit a portion of the row select driver circuit may be damaged or such defects as bad transistors, shorted lines, open lines, interlayer shorts, and other causes can lead to the malfunctioning all or a portion of the corresponding pixel row lines. In these cases of failure, a redundant row select driver circuit can compensate for these otherwise irreversible damages.
One such redundant scheme is disclosed in a patent to Asada, U.S. Pat. No. 5,111,060. Asada discloses providing a redundant or spare circuit element each and every primary circuit element. Asada then disclose electrically connecting a redundant circuit to a corresponding stage output where the primary circuit element is defective. This connection is made by such means as laser welding and the defective circuit is then electrically disconnected from the stage output. This means of providing redundant circuitry for a display requires having a complete duplicate row driver circuit that takes up valuable space on the substrate and in practice the vast majority of redundant circuits will never be used. It would be advantageous to provide a system wherein a number of redundant circuit elements could be provided that is less than the total number of primary circuit elements for driving the rows of a liquid crystal display. This would reduce the cost of manufacturing the display and be more efficient and cost effective in manufacturing.